Transcript Introduction CS 239 Security for Networks and System

Protecting Interprocess
Communications
• Operating systems provide various kinds of
interprocess communications
– Messages
– Semaphores
– Shared memory
– Sockets
• How can we be sure they’re used properly?
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IPC Protection Issues
• How hard it is depends on what you’re
worried about
• For the moment, let’s say we’re worried
about one process improperly using IPC to
get info from another
– Process A wants to steal information
from process B
• How would process A do that?
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Message Security
Process A
Gimme your
secret
Process B
That’s probably
not going to work
Can process B use messagebased IPC to steal the secret?
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How Can B Get the Secret?
• He can convince the system he’s A
– A problem for authentication
• He can break into A’s memory
– That doesn’t use message IPC
– And is handled by page tables
• He can forge a message from someone else to get
the secret
– But OS tags IPC messages with identities
• He can “eavesdrop” on someone else who gets the
secret
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Can an Attacker Really
Eavesdrop on IPC Message?
• On a single machine, what is a message send,
really?
• A message is copied from a process buffer to an
OS buffer
– Then from the OS buffer to another process’
buffer
– Sometimes optimizations skip some copies
• If attacker can’t get at processes’ internal buffers
and can’t get at OS buffers, he can’t “eavesdrop”
• Need to handle page reuse (discussed earlier)
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Other Forms of IPC
• Semaphores, sockets, shared memory, RPC
• Pretty much all the same
– Use system calls for access
– Which belong to some process
– Which belongs to some principal
– OS can check principal against access control
permissions at syscall time
– Ultimately, data is held in some type of memory
• Which shouldn’t be improperly accessible
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So When Is It Hard?
Always possible that there’s a bug in the
operating system
– Allowing masquerading, eavesdropping, etc.
– Or, if the OS itself is compromised, all bets
are off
2. What if the OS has to prevent cooperating
processes from sharing information?
1.
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The Hard Case
Process A
Process B
Process A wants to tell the secret to process B
But the OS has been instructed to prevent that
A necessary part of Bell-La Padula, e.g.
Can the OS prevent A and B from colluding
to get the secret to B?
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OS Control of Interactions
• OS can “understand” the security policy
• Can maintain labels on files, process, data
pages, etc.
• Can regard any IPC or I/O as a possible leak
of information
– To be prohibited if labels don’t allow it
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Covert Channels
• Tricky ways to pass information
• Requires cooperation of sender and
receiver
– Generally in active attempt to
deceive system
• Use something not ordinarily regarded
as a communications mechanism
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Covert Channels in Computers
• Generally, one process “sends” a covert
message to another
– But could be computer to computer
• How?
– Disk activity
– Page swapping
– Time slice behavior
– Use of a peripheral device
– Limited only by imagination
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Handling Covert Channels
• Relatively easy if you know what the
channel is
– Put randomness/noise into channel to
wash out message
• Hard to impossible if you don’t know
what the channel is
• Not most people’s problem
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File Protection
• How do we apply these access protection
mechanisms to a real system resource?
• Files are a common example of a typically shared
resource
• If an OS supports multiple users, it needs to
address the question of file protection
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Unix File Protection
• A model for protecting files developed in the
1970s
• Still in very wide use today
– With relatively few modifications
• To review, three subjects
• Owner, group, other
• and three modes
– Read, write, execute
– Sometimes these have special meanings
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Setuid/Setgid Programs
• Unix mechanisms for changing your user identity
and group identity
• Either indefinitely or for the run of a single
program
• Created to deal with inflexibilities of the Unix
access control model
• But the source of endless security problems
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Why Are Setuid Programs
Necessary?
• The print queue is essentially a file
• Someone must own that file
• Can’t make it world-writable
– Or people will delete each other’s jobs
• So how will people put stuff in the print queue?
• Typical Unix answer is run the printing program
setuid
– To the owner of the print queue
– Program only allows limited manipulation of
the queue
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Why Are Setuid Programs
Dangerous?
• Essentially, setuid programs expand a
user’s security domain
• In an encapsulated way
– Abilities of the program limit the
operations in that domain
• Need to be damn sure that the
program’s abilities are limited
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Some Examples of Setuid
Dangers
• Setuid programs that allow forking of a new shell
• Setuid programs with powerful debugging modes
• Setuid programs with “interesting” side effects
– E.g., lpr options that allow file deletion
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Encrypted File Systems
• Data stored on disk is subject to many risks
– Improper access through OS flaws
– But also somehow directly accessing the disk
• If the OS protections are bypassed, how can we
protect data?
• How about if we store it in encrypted form?
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An Example of an Encrypted File
System
Issues for
encrypted file
systems:
Ks
Transfer
Sqzmredq
#099 to
$100
sn
lx
my
rzuhmfr
savings
zbbntms
account
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When does the
cryptography occur?
Where does the
key come from?
What is the
granularity of
cryptography?
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When Does Cryptography Occur?
• Transparently when a user opens a file?
– In disk drive?
– In OS?
– In file system?
• By explicit user command?
– Or always, implicitly?
• How long is the data decrypted?
• Where does it exist in decrypted form?
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Where Does the Key Come From?
•
•
•
•
•
•
Provided by human user?
Stored somewhere in file system?
Stored on a smart card?
Stored in the disk hardware?
Stored on another computer?
Where and for how long do we store
the key?
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What Is the Granularity of
Cryptography?
•
•
•
•
An entire file system?
Per file?
Per block?
Consider both in terms of:
– How many keys?
– When is a crypto operation applied?
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What Are You Trying to Protect
Against With Crypto File Systems?
• Unauthorized access by improper users?
– Why not just access control?
• The operating system itself?
– What protection are you really getting?
• Data transfers across a network?
– Why not just encrypt while in transit?
• Someone who accesses the device not using the
OS?
– A realistic threat in your environment?
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Full Disk Encryption
• All data on the disk is encrypted
• Data is encrypted/decrypted as it
enters/leaves disk
• Primary purpose is to prevent improper
access to stolen disks
– Designed mostly for laptops
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Hardware Vs. Software Full Disk
Encryption
• HW advantages:
– Probably faster
– Totally transparent, works for any OS
– Setup probably easier
• HW disadvantages:
– Not ubiquitously available today
– More expensive (not that much, though - ~$90
vs. ~$60 for 320Gbyte disk)
– Might not fit into a particular machine
– Backward compatibility
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An Example of Hardware Full Disk
Encryption
• Seagate’s Momentus 7200 FDE product line
• Hardware encryption for entire disk
– Using AES
• Key accessed via user password, smart card, or
biometric authentication
– Authentication information stored internally on
disk
– Check performed by the disk itself, pre-boot
• 300 Mbytes/sec sustained transfer rate
• Primarily for laptops
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Example of Software Full Disk
Encryption
• Vista BitLocker
• Doesn’t encrypt quite the whole drive
– Need unencrypted partition to hold bootstrap
stuff
• Uses AES for cryptography
• Key stored either in special hardware or USB
drive
• Microsoft claims “single digit percentage”
overhead
– One independent study claims 12%
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